The present invention relates to a light emitting device, and more particularly, to a heterogeneously integrated light emitting device which may be directly powered by a high DC voltage or by an AC voltage for general lighting, indication or display purposes.
The advances in III-Nitride semiconductors (including GaN, InN, AlN and their alloys) based light emitting diodes (“LEDs”) is dramatically changing the lighting technology with a new lighting paradigm. LEDs, which have been until recently mainly used as simple indicator lamps in electronics and toys, now have the great potential to replace incandescent light bulbs in many applications, particularly those requiring durability, compactness, and/or directionality (e.g., traffic, automotive, display, and architectural lighting). Compared with the conventional lighting, semiconductor LED based solid state lighting (“SSL”) has the benefits of being more energy efficient with less power consumption, having a longer operational life with reduced maintenance costs, being vibration-resistant, having a vivid saturation color, and the added benefit of a flexible lighting design. It has been estimated that by the year 2025 the electricity saved in the United States by using solid state lighting would be approximately 525 trillion watt hours per year, or $35 billion a year. Additionally, the human visual experience would be enhanced by independently tuning the light intensity and colors of the LEDs.
The conventional LED, depending on the semiconductor materials, operates at a very low DC voltage (roughly between 1V and 5V) and a limited current (˜20 mA) with very low luminance, only suitable for indication purposes. To achieve a high luminance for general lighting applications, two methods have been adopted. In the first method the LED still operates at a low DC voltage, but with a very high DC current (<100 mA) to achieve a high luminance. The so-called power LED requires a bulky voltage transformer, an electronics controller and driver to power the LED. In a second method many LEDs are integrated on the same chip with a serial interconnection to achieve one light emitting device, which can directly run under a high DC input voltage. Depending on the integrated LED numbers, the operational voltage may be 12V, 24V, 110V, by, 240V, or even higher. Additionally, with two current paths the high voltage light emitting device may also operate directly at 110/120V or 220/240V AC. This highly integrated high voltage LED device has a size of between hundreds of microns to tens of millimeters, which is completely different from the disclosure in U.S. Pat. No. 6,787,999, in which many discrete packaged LED lamps are serially soldered on a printed circuit board. Other conventional devices have used serially connected packaged LEDs soldered together on a PCB board to form a bulk LED cluster for high voltage applications.
The concept of an integrated single chip LED device which operates under a high DC and/or AC voltage (high voltage DC/AC LED) unfolds a new paradigm for LED applications in lighting, indication and displays. As one example, the high voltage LED may be directly powered by the 110V power grid without any voltage transformer. If the high voltage LED is packaged with a standard Edison or European screw base, it may be directly screwed into a standard light bulb fixture for indoor or outdoor lighting. FIGS. 1 and 2 illustrate the principle to build such a device by directly integrating many LEDs together on a single chip. As illustrated, an InGaAlN LED is grown on a sapphire substrate or other insulating substrate, for example. A prior art conventional low voltage DC LED is generally indicated by reference numeral 10. LED 10 includes a substrate 12, an n-type semiconductor layer 14, a light emission region 16, and a p-type semiconductor layer 18, a p-contact 20, an n-contact 22, and a current spreading layer 24. As illustrated in FIG. 2, a prior art integrated high voltage LED device is generally indicated by reference numeral 26. A number of LEDs serially connect by connecting the p-layer 18 of one LED 10 with the n-layer 14 of the adjacent LED with an interconnection metal layer 28. The integrated LED 26 has two terminals 30 and 32 for connection to an input voltage. Light 34 is extracted from the semiconductor epilayer 18 through the semi-transparent current spreading layer 24.
Several problems with prior art integrated LEDs include inefficient light extraction, thermal dissipation, and low product yield and reliability. Each individual LED 10 has to be isolated from the others by etching through the n-type semiconductor layer 14 to the insulating substrate 12 or to an insulating growth layer (buffer, epilayer, etc.). For InGaAlN-based LEDs, this etching depth is approximately from 2 μm to 6 μm. The deep trenches 36 provide technical challenges for depositing the metal layers 28 to interconnect each LED 10. An inconsistent or thin metal layer 28 may cause leakage or disconnection at the trench side walls 38, which may result in product performance, yield and reliability degradation.
For an InGaAlN based LED device sapphire is the most common substrate and is also the best option for a high voltage LED device because of its high insulation property. If SiC or Si is used as the substrate an insulation buffer layer will be required. Unfortunately, sapphire has a very low thermal conductivity and the limited thermal dissipation degrade the high voltage (and high power) LED device performance and lifetime. Another drawback for the prior art is that the light is extracted from the epilayer device side and a significant portion of the light is blocked and absorbed by the metal layers, including the p-contact 20, n-contact 22, metal layer 28 and the current spreading layer 24 limiting the light emitting efficiency.